Recently, signal processing speed of electronic equipment has been getting faster and faster. Consequently, transmission speed of signals passing through printed wiring boards has been increasing as well. In high-speed signal transmission, it is necessary to match characteristic impedance of signal lines, and mismatch will cause signal reflection, resulting in losses.
Generally, the characteristic impedance is 50Ω in the case of a single-ended channel, and 50Ω per signal line in the case of a differential channel. To obtain desired impedance, printed wiring boards which handle high-speed signals use a microstrip line in which a signal line 1 and grounding conductor 2 oppose each other across a substrate 3 as shown in FIG. 8.
Normally, microstrip lines use a solid grounding conductor. That is, an entire surface without a signal line is structured to be a ground plane. However, in the case of flexible printed wiring boards, substrates are 25 μm thick in many cases, and generally as thin as 12.5 to 50 μm. This increases capacitance between the signal line and grounding conductor, resulting in reduced characteristic impedance of the microstrip line.
For example, a microstrip line with a solid grounding conductor and with a signal line width of 122 μm has characteristic impedance of 50Ω when 100-μm-thick glass/epoxy prepreg (with a relative dielectric constant of 4) is used, but the characteristic impedance drops to 8Ω or less when 25-μm-thick polyimide (with a relative dielectric constant of 3.3) is used.
To reduce the capacitance between the signal line and solid grounding conductor and ensure characteristic impedance of 50Ω using the same polyimide material, it is necessary to reduce the signal line width to as small as 17 μm. Such a signal line can be produced if a thin conductor is used, but the narrow line width and small thickness will increase DC resistance.
Also, due to the narrow signal line width, variations in the width of formed signal lines have a significant impact, making it difficult to obtain desired impedance accurately. Thus, a square-meshed (see Patent Document 1) or diamond-meshed (see Patent Document 2) grounding conductor is often used.
However, since characteristic impedance varies depending on whether a signal line is located on opening or conductive portions of a meshed grounding conductor, the use of the meshed grounding conductor will cause impedance mismatch, resulting in signal reflection or losses.
Also, when a signal line is laid out in a bent form, position of the signal line relative to the mesh changes before and after the signal line is bent. This causes characteristic impedance mismatch.
To deal with this, a technique has been proposed for making a meshed shape conform to the curved shape of the signal line (see Patent Document 3). Also, a technique has been proposed for controlling opening size of the mesh in inner and outer regions of the curved shape or controlling signal line width (see Patent Document 4).
However, width of the wire-type grounding conductors 5 is always equal to (as shown in FIG. 9(a)), or larger than, width of signal lines 4. If it is assumed that there is no misalignment between the wire-type grounding conductors and signal lines, since the wire-type grounding conductors are equal in area to the opposing signal lines, the capacitance between the signal lines and grounding conductors is the same as in the case of a solid grounding conductor.
Thus, when a thin insulating layer is put between the signal lines and grounding conductors as in the case of a flexible printed wiring board, it is difficult to transmit large current or control impedance accurately.
Also, misalignment can occur in an actual manufacturing process as shown in FIG. 9(b), causing changes in the overlapping areas of signal lines 6 and grounding conductors 7 located opposite to each other. This makes it impossible to obtain desired impedance.
Patent Document 1: Japanese Patent Laid-Open No. 2000-114722
Patent Document 2: Japanese Patent Laid-Open No. 2006-147837
Patent Document 3: Japanese Patent Laid-Open No. 2006-173310
Patent Document 4: Japanese Patent Laid-Open No. 2000-077802
Patent Document 5: Japanese Patent Laid-Open No. 2001-085805